Slight tweak of notification message
Slight tweak of notification message

Old message wasn't clear if the number of additional new messages
was in addition to the one being reported or total new messages.
Additional new messages is a more interesting value, so this changes
the value to additional and the message to clarify that.

glebihan/geary,martinra/geary,AtticHacker/geary,robol/geary,kpiwko/geary,githubnemo/geary,githubnemo/geary,emersion/geary,aatishnn/geary,mjog/geary,rschroll/geary,rschroll/geary,mjog/geary,mpdeimos/geary,emersion/geary,tshikaboom/geary,emersion/geary,aatishnn/geary,glebihan/geary,aatishnn/geary,abn/geary,jimbonator/geary,geekvigilante/geary,mjog/geary,AtticHacker/geary,AtticHacker/geary,tshikaboom/geary,glebihan/geary,robol/geary,tshikaboom/geary,jimbonator/geary,martinra/geary,jimbonator/geary,martinra/geary,MohamedIM/geary-unofficial,githubnemo/geary,robol/geary